1. Technical Field
This invention relates generally to the highly miniaturized springs. More particularly, this invention relates to a family of miniaturized contact springs and a family of methods for increasing the yield strength and fatigue strength of these springs.
2. Description of the Prior Art
Miniaturized springs have been widely used as electrical contacts to contact pads or I/O terminals on integrated circuits, PCB-s, interposers, space transformers and probe chips for purposes such as testing, burn-in and packaging because even arrays of such miniaturized springs can be fabricated with a pitch of less than 10 μm. A miniaturized stress metal film spring, usually patterned by photolithography, comprises a fixed portion, also called anchor portion, attached to a substrate and a lifted portion, also called free portion, initially attached to the substrate, which upon release extends away from the substrate forming a three dimensional structure as a result of inherent stress gradient in the spring. Typically, the stress gradient in a film is produced through sequential deposition of a plurality of thin film layers by sputtering or electroplating under different process conditions. A typical embodiment of stress metal spring is schematically shown in FIG. 1a, which comprises an anchor portion 101 associated with an electrical contact or terminal 102 attached to a substrate or electrical component 103, and a free portion 104 with a spring tip 105. Examples of such structures are disclosed in U.S. Pat. No. 5,613,861 (Smith) and application PCT/US00/21012 (Chong, Mok).
Other types of springs include discrete springs, fabricated individually or in a group and subsequently mounted on a substrate, such as those used in wafer test or burn-in assembly, or those comprising integrated solid-state devices such as semiconductor devices. Still other types of such springs are those cantilever types of springs, which are fabricated en masse on a substrate using photolithography, as mentioned in the patent literature such as PCT 01/48818, PCT WO97/44676, U.S. Pat. No. 6,184,053, and PCT WO01/09952. Some of these springs are fabricated individually or in a group on a sacrificial substrate and then mounted onto substrates used in the wafer test or burn-in assemblies, or onto those comprising semiconductor devices. FIG. 1b is a schematic cross view of a typical photo-lithographically patterned freestanding cantilever spring fabricated on sacrificial layers, which comprises a base region 201 at one end that is attached to an electrical contact pad 202 of a substrate 203, a contact tip region 204 at the other end of the spring, and the central main body 205 of the spring connecting the base 201 and the contact tip region 204. The problem in this kind of springs is that they are too long. Shorter and smaller springs are desirable for testing and burn-in of some of the current and next generations of integrated circuits, which comprise contact terminal pads with very small pitch, 20–50 μm, for example.
Methods of fabricating shorter springs using photo lithographically processes to add thicker metal coatings have been defined in the patent literature. One method is described in application WO 01/48870. This method uses electroplated photo resist to allow metal to be plated on the top of a free standing spring. However, at the dimensions needed to probe ICs with pad pitches below 150 μm the freestanding springs have insufficient strength to hold backside photo resist without significantly reducing the probe height required for compliance. Any non-uniformity in the photo process also translates to non-uniform spring heights that cannot meet the uniformity requirements necessary to stay on the IC pads while testing.
The method described in application (WO 01/48870)) also has an additional problem in controlling lift height after plating. One of the purposes of having a freestanding spring is to provide a framework or structure to support the thicker plated metal. If one plates a spring on only one side, the spring curves to a different lift height based on the stress in the plated film. If the film is tensile it curves up and if it is compressive it pushes it down. Both of these stress conditions are difficult to control for the tolerances and spring lift uniformity needed to test ICs. In addition, compressive springs are stronger than tensile springs and the spring with a compressive plated film loses lift height to the point that there is not enough compliance for it to still be a useful probe. There is also a limit to how high a freestanding spring can be lifted prior to plating to compensate for this compression effect. The probe needs to make contact to the IC electrical pad at an angle less than 90 degrees. Increasing the lift height tends to cause the spring to wrap around itself creating a 360 degree circle to the substrate. As a result, the process taught by this patent application does not meet the requirements for controlling uniformity of the lift height of arrays of springs required for IC testing.
One method to build the probe in application WO 01/48870 is to assemble a tip on the plated spring and assemble the spring fabricated on a sacrificial substrate to a second interconnect substrate. The assembly process adds positional placement errors and is more expensive to manufacture than a fully integrated connection button tip as described in the invention herein.
Another method described in patent number U.S. Pat. No. 6,528,350 keeps the photoresist coating, i.e. mask, off the spring, and uses a release layer island to allow plating of the freestanding portion of the spring. For cases where the release mask stops adjacent to the base (anchor portion) of the spring and does not extend along the base of the spring, the thickness and width of the free portion of the spring close to the base become much larger upon plating compared to the base region. As a result, the freestanding portion of the spring is mechanically weaker in the vicinity of the base region. Because the bending moment is the highest in this region, upon application of a force to the spring tip during IC test, the springs fracture early and therefore can not meet the probe lifetime requirement needed in IC manufacturing lines. For the other method described in U.S. Pat. No. 6,528,350, where the photoresist mask does not cover the freestanding portion of the spring as well as part of the anchor portion during plating, there still is a discontinuity in the width which tends to fracture. The mask alignment and control of spring release process also pose serious problems resulting in uneven plating and a variation in lift uniformity. Another major problem in this process arises from the high resistivity of the relatively thin release layer, as well as the stress metal film, through which the plating current flows. The current density varies widely with the distance from the power connection points at the edge of the substrate. As a result, the plated film characteristics, e.g. microstructure, thickness, stresses etc., vary widely at different areas of the spring. As a result, this process does not produce arrays of springs with reasonably uniform and controlled properties, such as lift height that is essential for effective IC testing.
The invention herein comprises several means to circumvent the problems associated with the above two methods and provides solutions that allow manufacturing of arrays of springs suitable for meeting the stringent requirements of wafer level IC testing. Among other things, the invention allows fabrication of arrays of springs with reasonably uniform lift height and properties, as well as durability. For example, it teaches the practice of enveloping the entire spring core, both freestanding and anchor portions, with electrodeposited films with a balanced stress that allows maintenance of spring heights with appropriate uniformity after electrodeposition. In another teaching, it shows a method to plate the springs selectively without the use of any photoresist mask.
The miniaturized contact springs are subjected to a large number of contact operations during testing which subject the springs to various levels of stresses including cyclic stresses. Also, in packages that use contact springs to join two components, such as chips and chip carriers, the springs are subjected to stresses during testing and operation. The springs are required to withstand such stresses without failure. However, we have observed that the miniaturized springs, such as those with a size of around 400 μm×60 μm×20 μm, start to fail, i.e. being plastically deformed and/or fractured, typically after 10,000 touchdowns, where the contact force exceeds about 1 gf. A major reason of the failure is that the resulting alternating stresses exceed fatigue strength of the spring material. The fatigue strength indicates the alternating stress level at which a material can withstand a specified number of cycles. It is typically some fraction of a material's yield strength, which corresponds to the onset of plastic deformation, i.e. instantaneous permanent deformation. Because a force exceeding about 1 gf is usually required to make good reproducible contacts on aluminum with low contact resistance, as observed in our experiment, the resistance of the springs to failure must therefore be significantly increased to improve the performance and quality of the springs. Springs with larger cross-sections can withstand similar or larger force without failure because the resulting stresses are lower, but they limit the pitch at which springs can be built.
For some operations, such as the burn-in of devices, contact springs are required to make contacts with the device terminals at an elevated temperature, for example around 100-C. Such contacts may also be required to allow passage of a relatively high current, for example 250–500 mA, during the operation. Under this condition the contact resistance should be quite low, for example 0.1 milli-ohm, so that the contact tip region of the spring may not get damaged by overheating. One way to achieve the low contact resistance is to increase the contact force, by increasing the thickness of the springs. However, a higher contact force increases the stress developed in the body of the spring, particularly near the base region, and thus increases the probability of early spring failure during repeated touchdowns.
Furthermore, the materials from electrical contact pads or terminals tend to adhere to the spring tips during repeated contacts. In cases where the adherence of the pad material to the spring tips increases the contact resistance, or the pad material readily forms tenacious compounds upon exposure to the ambient condition, the electrical contacts are degraded after repeated touchdowns. This also shortens the springs' lifetime. Thus contact tip structure should preferably be comprised of materials that do not have strong adherence to the contact pads or terminals.
Therefore, what is desired is a mechanism for maximizing yield strength and fatigue strength of the miniaturized springs within the miniaturization requirement.
What is further desired is a mechanism to minimize adhesion of the contact pad materials to the spring tips upon repeated contacts without substantially affecting reliability and electrical conductivity of the springs.
A method to fabricate springs with high resistance to compliant stress that results in uniform spring height and provides for a durable tip structure is desired.